Page 205 - Chalcogenide Glasses for Infrared Optics

CHAPTER 8

IR Imaging Bundles

Made from

Chalcogenide Glass

Fibers

8.1 The Stacked Ribbon Method
It is worth repeating that the optical and physical properties of chal-
cogenide glass fibers are vastly different from those of the silicate-
based fibers. Generally, there are two methods for making imaging
bundles from glass fibers: (1) the ribbon stacking method the original
method used to produce bundles, and (2) the method developed for
silicate fibers over many years, termed the leachable bundle method. In
this second method, glass clad rods are clad again with a third glass
which is acid-soluble. The rods are then fused in a rigid bundle,
heated, and drawn down in size together to a smaller diameter,
perhaps one-tenth in diameter, but increased in length. The rods are
then cut into convenient lengths and fused again, and the process is
repeated until the desired core size is reached. The rods are next cut
into imaging bundle lengths. The ends are protected by an acid-resistant
coating such as wax, and the bundle is placed in an acid bath to
dissolve away the third glass. The fibers are thus separated from each
other, providing flexibility for the bundle. Bundles prepared in this
manner have a near-perfect fiber arrangement. Image performance is
near perfect as well. The bundle characteristics regarding uniformity
of core size and clad thickness relative to core are well controlled
when the rods are made before the first draw. Unfortunately, chalco-
genide glass fibers are not suited for this process due to the relative
volatility of the chalcogenide glasses and their chemical inertness to
acids. Chalcogenide glasses are only attacked by strong alkali. Also as

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